I am having a session problem in Asp.net

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ASP.NET

I am having a session problem when a user is logged in and then logs out. They can hit the back button to regain access to the previous page they were on. Here is my two codes I have.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

public partial class Login : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {

        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["PassConnectionString"].ConnectionString);
        con.Open();

        if (true)
        {
            SqlCommand level = new SqlCommand("select AccessLevel, Password from TableSecurity where EmailAddress = @EmailAddress AND Password = @Password", con);
            level.Parameters.Add(new SqlParameter("EmailAddress", TextBoxEA.Text));
            level.Parameters.Add(new SqlParameter("Password", TextBoxPW.Text));

            SqlDataReader reader = level.ExecuteReader();
            DataTable dt1 = new DataTable();
            dt1.Load(reader);

            foreach (DataRow dr1 in dt1.Rows)
            {
                int returnedLevel = Convert.ToInt32(dr1[0].ToString());
                if (returnedLevel == 1)
                {
                    Response.Redirect("CEOPage.aspx");
                }

                else if (returnedLevel == 2)
                {
                    Response.Redirect("IALOPage.aspx");
                }
            }
        }
        con.Close();
    }
    
}

This is the logout code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class IALOPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Session["New"] = null;
        Response.Redirect("Login.aspx");
    }
}

What did I do wrong or what did I miss? Please help!!!